Abe Pazos aka hamoid has produced a processing library that makes it very easy to export processing sketches as videos (essentially it is a wrapper for ffmpeg, so you could easily achieve more customised videos yourself, but using the library makes it quick and easy). Get the library here or from github. For JRubyArt just follow the instructions for installing the vanilla processing version, then you can ‘load the library’ in the normal way, not forgetting to ‘include_package’. For your convenience here is the basic example I have translated for you.

basic_example.rb (JRubyArt Sketch)

load_library :VideoExport
include_package 'com.hamoid'

# Press 'q' to finish saving the movie and exit.

# In some systems, if you close your sketch by pressing ESC,
# by closing the window, or by pressing STOP, the resulting
# movie might be corrupted. If that happens to you, use
# video_export.end_movie like you see in this example.

# In some systems pressing ESC produces correct movies
# and .end_movie is not necessary.
attr_reader :video_export

def settings
  size(600, 600)

def setup
  sketch_title 'Basic Example'
  @video_export = VideoExport.new(self)

def draw
  rect(frame_count * frame_count % width, 0, 40, height)

def key_pressed
  return unless (key == 'q')